The invention relates to an array of lenses for use with a thermal radiation detector to monitor thermal radiation arriving from a fan of separate directions. The pole of each lens and the detector define one detection direction in the fan, the array being formed with the poles of lenses being aligned. Herein, the pole of a lens is defined as the intersection of the optical axis of the lens with the lens element. Hence, a ray incident at the pole passes undeviated through the lens. The line joining the pole and the detector thus defines a detection direction.
In apparatus for monitoring thermal radiation, and in particular in an intruder alarm apparatus, the fan of separate directions should cover at least 90 degrees of azimuth, and preferably 120.degree., and up 45 degrees in elevation from a horizontal direction downward. Such an apparatus placed high in the corner of a rectangular room, for example, will effectively cover the whole volume of the room.
Desirably, the radiation collection efficiencies of all the lenses in the array are equal so that all directions in the fan are equally covered. Monitoring apparatus is known in which the lenses are provided on a curved surface, the curve being centered on the detector. Each lens in the array is then normal to its detection direction in at least one azimuth. Each lens then forms its image on its optical axis and aberrations are minimized.
It is known to provide the array of lenses by a moulding operation performed on a sheet of plastic material and to form the lenses as Fresnel lenses thereby minimizing the thickness of the sheet. However, such a thin curved sheet protruding from the apparatus is vulnerable to damage. A flat or quasi-flat array of lenses is desirable in fabrication and in the fixing arrangements of the array to the apparatus. A flat lip on the edges of the array can more easily form part of the external wall of a rectangular housing for the apparatus.
Flat arrays of lenses provided on a sheet are known from U.S. Pat. No. 3,547,546 where the lenses are zone plates. However, if zone plates are used 45.degree. off-axis, as is necessary in the present intruder alarm apparatus, the image quality would be greatly degraded and radiation loss due to reflection would reduce the efficiency of such a lens. If the lenses of the array are Fresnel lenses, 45.degree. off-axis operation would again produce loss of radiation by reflection at the outer surfaces of the lens and also by total internal reflection within the Fresnel elements of the lens.
Arrays of moulded Fresnel lenses are also known from U.S. Pat. No. 4,321,594. But therein, to achieve a fan of separate directions, the moulded sheet is substantially curved or bent and hence protrudes from the apparatus indesirably as previous noted.